The ultimate goal of the proposed studies is characterization of the role of the medial efferent system in human perception of sound. It combines psychophysical and physiological measurements in the same subject, from both humans and monkeys. The PI proposes the development of psychophysical tasks that reflect known efferent processes. The characteristics of the distortion product otoacoustic emission (DPOAE) will be used to define the stimulus conditions for the specific aims. Based on psychophysical and anatomical studies of the two species, use of identical stimulus conditions allows direct comparison between the two species. The first study (Aim I) is designed to determine the relationship of "central masking" and efferent reduction of the DPOAE, both effects produced by contralateral acoustic stimulation. Psychophysical suppression will be measured parametrically by target frequency, contralateral noise bandwidth, intensity and frequency. In human subjects, the contralateral efferent effect on the DPOAE will determine the efferent involvement. Similar experiments, using identical stimulus conditions, will be performed with the monkey as the subject. The efferent involvement in perception, measured from the human, will be inferred from monkeys with discrete lesions. The experiments performed in Aim I will aid in the specification of stimulus conditions of subsequent Aims. In Aim II the PI proposes to characterize the role of the medial olivocochlear (MOC) in possible improvement in the perception of signals in noise. These studies are intended to determine the conditions in which enhanced perceptual acuity occurs. Aim III focuses on the role of the MOC in psychophysical adaptation. Recent physiological experiments suggest that some aspects of adaptation are related to MOC function. The DPOAE in response to sustained stimulation shows a rapid adaptation that is eliminated when the uncrossed efferent system is cut. The PI proposes to test, in parallel, psychophysical and physiological adaptation in humans and monkeys. The time course of the adaptation for each response measure will be compared. The involvement of the MOC in these two response measures will be evaluated by MOC lesions in the monkeys. The goal of Aim IV is to determine the role of the MOC tracts in auditory "selective" attention. Psychophysical and physiological evidence suggests that a control mechanism exists that can suppress extraneous or unattended signals. Parallel psychophysical and physiological studies in humans and monkeys are proposed to replicate the previous studies. The role of selective attention will be addressed by discrete MOC lesions and monitoring performance changes.